Question

Which one of the following mixed sulphates is not an alum?
(A) ${K_2}S{O_4}.A{l_2}{\left( {S{O_4}} \right)_3}.24{H_2}O$
(B) ${K_2}S{O_4}.C{r_2}{\left( {S{O_4}} \right)_3}.24{H_2}O$
(C) $N{a_2}S{O_4}.Fe{\left( {S{O_4}} \right)_3}.24{H_2}O$
(D) $CuS{O_4}.A{l_2}{\left( {S{O_4}} \right)_3}.24{H_2}O$

Answer 1

In this question, we have to identify which of the given options is not an alum . Alums are double salts , so based on this criteria we will identify which compound is not an alum .

Complete step by step answer:.
Alum is an example of double salt . Double salts are formed by the combination of two or more stable compounds in stoichiometric ratio . Hence , they are called addition or molecular compounds.
When double salts are dissolved in water they dissociate into ions completely .
In the given options all are alums except $CuS{O_4}.A{l_2}{\left( {S{O_4}} \right)_3}.24{H_2}O$
Double salt of copper sulphate and aluminium sulphate is not an alum and also it is toxic in nature .

So, the correct answer is Option D.

Additional information :
Double salt have the following properties :
They usually contain two simple salts in equimolar proportions .
They exist in the solid state . In aqueous solution , they dissociate completely into ions .
They are ionic compounds and do not contain any coordinate bond .
The properties of the double salt are the same as those of its constituent compounds .
In a double salt , the metal ions show their normal valency
A double salt loses its identity in the solution .

Note: We often get confused between coordination compounds and double salts as they both are formed by the combination of two or more stable compounds in stoichiometric ratio . But they differ in the fact that when double salts are dissolved in water they dissociate into ions completely whereas coordination compounds do not dissociate completely .